Tip #1-don't buy Pro Tools that claims "no iLok required" as that is cracked software and illegal(seller should be reported)
Tip #2-look for a seller with lots of great feedback and very few or no negative feedback
Tip #3-read the listing over carefully(and maybe more than once) to make sure you are getting(hopefully) new and unregistered software.
Tip #4-be aware of EVERYTHING that you will need to run Pro Tools, including:
1-a compatible computer and OS
2-an iLok 2 and iLok account(free to set up)
3-The PT asset has to actually be loaded onto the iLok(which is done by logging onto your iLok account and redeeming a code, then dragging the license onto the iLok)
4-a suitable "audio interface". Simple playback can be done with Core Audio(Mac) and an ASIO compatible sound card(PC), but to do any serious recording and mixing, you are better off with a "real" interface. Look into MOTU, RME, Focusrite(and Apogee if you run a Mac) My 2 cents only, but I don't recommend M-Audio interfaces...

Another one, don't buy if the item is an iLok with a PT license already on it. This means the license isn't in your account. While you can still use the software, you're out of luck if the iLok breaks or if you ever want to upgrade or transfer.